Abstract
Abstract The Neil Gehrels Swift Observatory carried out prompt searches for gravitational-wave (GW) events detected by the LIGO/Virgo Collaboration (LVC) during the second observing run (“O2”). Swift performed extensive tiling of eight LVC triggers, two of which had very low false-alarm rates (GW170814 and the epochal GW170817), indicating a high confidence of being astrophysical in origin; the latter was the first GW event to have an electromagnetic counterpart detected. In this paper we describe the follow-up performed during O2 and the results of our searches. No GW electromagnetic counterparts were detected; this result is expected, as GW170817 remained the only astrophysical event containing at least one neutron star after LVC’s later retraction of some events. A number of X-ray sources were detected, with the majority of identified sources being active galactic nuclei. We discuss the detection rate of transient X-ray sources and their implications in the O2 tiling searches. Finally, we describe the lessons learned during O2 and how these are being used to improve the Swift follow-up of GW events. In particular, we simulate a population of gamma-ray burst afterglows to evaluate our source ranking system’s ability to differentiate them from unrelated and uncataloged X-ray sources. We find that ≈60%–70% of afterglows whose jets are oriented toward Earth will be given high rank (i.e., “interesting” designation) by the completion of our second follow-up phase (assuming that their location in the sky was observed), but that this fraction can be increased to nearly 100% by performing a third follow-up observation of sources exhibiting fading behavior.
Highlights
In 2017, the Advanced Laser Interferometer Gravitationalwave Observatory and the Advanced Virgo detector collectively carried out the second observing run (“O2”) in search of gravitational-wave (GW) events from 2016 November 30 to August 25
Collaboration (LVC) were assigned parameters such as a falsealarm rate (FAR; characterizing the frequency at which noise with the same strength as the signal is expected to arise), whether the detected signal arose from a compact binary coalescence (CBC) or an unmodeled burst,25 and the estimated distance of the merger and the masses of the initial compact objects
Triggers with an FAR of less than one per month were announced to electromagnetic (EM) follow-up partners who had signed a memorandum of understanding with the LIGO/Virgo Collaboration (LVC)
Summary
In 2017, the Advanced Laser Interferometer Gravitationalwave Observatory (aLIGO; LIGO Scientific Collaboration et al 2015) and the Advanced Virgo detector (the VirgoScientific Collaboration; Acernese et al 2015) collectively carried out the second observing run (“O2”) in search of gravitational-wave (GW) events from 2016 November 30 to August 25. O2 resulted in the detection of GW170817, a binary neutron star (BNS) merger, which was the first GW event to have its EM counterpart (AT 2017gfo) detected (Abbott et al 2017). The results of both O1 and O2 are summarized in the Gravitational-Wave Transient Catalog of Compact Binary Mergers (GWTC-1; Abbott et al 2019b). To further our understanding of the physics of compact binary mergers, it is necessary to search for and study the EM counterparts to merger events following the detection of their GWs
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